1. Field of the Invention
The present invention relates to a multileaf collimator which adjusts radiation irradiated from a radiation generator toward a patient into a shape according to an affected part of the patient.
2. Description of the Related Art
One type of conventional multileaf collimator includes first and second groups disposed opposite to each other. Each group includes a plurality of leaf blocks arranged in a direction. The leaf blocks of the first group are oppositely disposed so as to be perpendicular to the leaf blocks of the second group The leaf blocks of the first group are connected to a drive mechanism having an electric motor serving as a drive source so that upon normal rotation of the motor, the leaf blocks of the first group are moved in such a direction that the leaf blocks of the first group are departed away from the leaf blocks of the second group. When the motor is reverse rotated, the leaf blocks of the first group are moved so as to come close to the leaf blocks of the second group. The leaf blocks of the second group are connected to a drive mechanism including an electric motor serving as a drive source. Upon normal rotation of the motor, the leaf blocks of the second group are moved so as to depart from the leaf blocks of the first group. On the other hand, the leaf blocks of the second group are moved close to the leaf blocks of the first group upon reverse rotation of the motor. The multileaf collimator includes detectors which detect current positions of the leaf blocks of the first and second groups respectively. Amounts of rotation of the motors are controlled based on the results of detection of current positions by the detectors respectively, whereby the spacing between the first and second leaf block groups is adjusted to a target configuration.
JP-A-2001-129104 discloses a multileaf collimator including a plurality of resistors serving as detectors and having different resistivity. The resistors are applied to a common leaf block so as to be arranged in a direction of movement of the leaf blocks. When the leaf blocks are moved in the oncoming or departing direction, the leaf blocks are moved while contacting paired electrodes. A voltage with a predetermined level is applied between the paired electrodes so that a current position of each leaf block is detected based on a magnitude of electric current flowing from one electrode to the other electrode. In the disclosed multileaf collimator, the resistance value varies according to variations in a mechanical contact resistance between the electrodes and the resistor. The resistance value also varies according to presence or absence of foreign matter such as dust. The resistance value further varies according to an external temperature. Accordingly, since current positions of the plural leaf blocks cannot be detected accurately, the spacing between the first and second leaf block groups cannot be set to a target configuration accurately.